1. Field of the Invention
The present invention relates to a diesel engine in which a premixed combustion is implemented, and more particularly to a diesel engine with an optimized ignition timing or premixing period in the premixed combustion.
2. Description of the Related Art
In diesel engines, fuel is typically injected close to a compression top dead center of the piston when the temperature and pressure inside the cylinder are high. In this case, the fuel (mixture) is ignited producing a flame during fuel injection and the combustion is maintained by supplying subsequently injected fuel into this flame. In such a conventional combustion mode, there is a portion of the initially injected fuel that is combusted after the ignition delay period, which causes a problem with NOx (nitrogen oxide). There is also a portion combusted in the combustion gas with an insufficient amount of oxygen, which causes a problem with smoke generation.
Accordingly, the applicants have suggested a diesel engine in which the fuel injection timing was before the compression top dead center and the mixture was ignited after the end of fuel injection (Japanese Patent Application Laid-open No. 2003-83119).
In such a diesel engine, the mixture is ignited once a certain interval elapses after the end of fuel injection. As a result, the mixture is sufficiently leaned and homogenized before the ignition. Therefore, a local combustion temperature decreases and the amount of released NOx is decreased. Furthermore, the emission of smoke is also inhibited because local combustion in an air-deficient state is avoided.
The combustion mode in which the mixture is ignited after the end of fuel injection will be referred to in the present specification as “premixed combustion”, and the period from the end of fuel injection to the ignition will be referred to as “premixing period”.
The premixed combustion is effective for improving the exhaust gas, but the following two problems are encountered when such combustion is realized.
(1) Difficulty of ensuring the premixing period: in the operation region with a comparatively large fuel injection quantity, the mixture is sometimes ignited during fuel injection even if the fuel is injected rapidly. In this case, the combustion mode of the mixture becomes identical to the conventional combustion. Therefore, there is no improvement with regards to the exhaust gas.
(2) Difficulty of controlling the ignition timing: in the conventional combustion mode in which the ignition starts during fuel injection, the ignition timing can be controlled by controlling the fuel injection timing. However, because a premixing period exists in the premixed combustion and the ignition timing is governed by external parameters such as the temperature inside the cylinder, pressure inside the cylinder, and air/fuel ratio, the ignition timing cannot be accurately controlled by merely controlling the fuel injection timing. In the case of inadequate ignition timing of the mixture in the premixed combustion, there is no improvement with regards to the exhaust gas. Moreover, the efficiency is degraded (fuel consumption is degraded) or combustion noise is generated. For example, if the ignition timing of the mixture is too early (even before the top dead center), thermal loss is increased and the compression induced by the piston is received after the ignition. Therefore, the temperature inside the cylinder rises, and there is a risk of generating NOx and degrading fuel consumption.
The Applicants have discovered that controlling an EGR (exhaust gas recirculation) ratio together with the fuel ignition timing is effective for resolving those two problems. In other words, if the EGR ratio is increased, the concentration of oxygen in the mixture decreases. Therefore, the premixing period is extended (ignition timing is delayed). Conversely, if the EGR ratio is decreased, the premixing period is shortened (ignition timing occurs earlier). Therefore, the premixing period and ignition timing can be optimally controlled by adequately controlling the EGR ratio in combination with controlling the fuel injection timing. This technology was not publicly known at the time the present application was filed and does not constitute prior art.
However, though the fuel injection timing can be comparatively accurately controlled by controlling the electric actuation timing of the injector, the EGR ratio is difficult to control stringently.
For example, because the EGR ratio is also influenced by parameters other than the control parameters relating to the EGR apparatus (for example, the EGR valve opening degree in the external EGR apparatus), the EGR ratio sometimes fluctuates under the effect of other factors even when the control parameters relating to the EGR apparatus are constant.
Furthermore, there is a substantial time lag from the instant the control parameters relating to the EGR apparatus have changed to the instant the EGR ratio of the mixture actually changes, due to the response delay of the EGR valve and the existence of the volume portion associated with the distance (intake path) from the EGR valve to the combustion chamber. For this reason, when the fuel injection timing and EGR apparatus are controlled, a temporary mismatch sometimes occurs between the two.
Thus, because the EGR ratio is difficult to control stringently, the ignition timing or premixing period of the mixture sometimes can become temporarily inadequate, creating a risk of degrading the exhaust gas or fuel consumption.